Kemplen was born into the film business, as his father produced silent films for traveling exhibition. At the age of 19, he edited The Frightened Lady, followed by the Boris Karloff horror film The Ghoul. He subsequently worked on propaganda films during WWII (while also serving as an auxiliary fireman), edited several entries in The Saint films, and entered a lengthy six-film collaboration with John Huston, including The Africn Queen and Beat the Devil (both with Robert Morley). Kemplen also worked with cinematographer Oswald Morris on several Huston entries and other films, and both men closed out their cinematic careers on the same two Jim Henson films. Kemplen's other credits included A Man for All Seasons and the musical re-make of Goodbye, Mr. Chips (with Peter O'Toole).

Kemplen's son William and grand-son Tom followed his footsteps, likewise distinguishing themselves as film editors.